Cognitive radio recognizes a surrounding wireless environment and optimizes communication parameters according to the wireless environment. One example of the cognitive radio is a case in which a plurality of radio systems share a frequency band. For example, there is a case in which a frequency band that can be preferentially used by one radio system (called a primary system) is secondarily used by another radio system (called a secondary system). It is discussed in Institute of Electrical and Electronic Engineers (IEEE) 802.22 to standardize a wireless regional area network (WRAN) corresponding to a secondary system which secondarily uses a frequency band (TV channel) licensed to a TV broadcasting system corresponding to a primary system.
When a secondary system secondarily uses a frequency band licensed to a primary system, it is necessary that the secondary system does not have any influence on services provided by the primary system. In order to avoid causing interference to the primary system, the secondary system uses a frequency band that is not temporally or spatially used by the primary system, or adjusts transmission power so that interference experienced by the primary system is below a tolerance level (see, e.g., patent literature 1).
A study has been actively carried out assuming a case in which, in one example of cognitive radio, a primary system is a TV broadcasting system and a secondary system is a cellular system. A frequency band which is not temporally or spatially used in a TV broadcasting system is called a TV white space (WS) (see, e.g., non-patent literature 1).
Some known examples of cognitive radio technology for specifying unused frequency bands include a Geo-location Database (GDB), frequency sensing, and a beacon (or Cognitive Pilot Channel (CPC)). Among these examples, two or more, e.g., a GDB and frequency sensing, or a GDB and a beacon, may be used in combination with each other. A GDB provides statuses of utilization of a shared frequency band (e.g., TV band) or information of a secondarily usable (i.e., unused) frequency band (e.g., TVWS), according to the geographical location.
For example, allocation of TVWS to a Long Term Evolution (LTE) system which is a cellular system is executed in the following procedure.
(1) An operation and management apparatus of an LTE system informs the GDB about information of a base station (i.e., evolved Node B (eNB)) which desires to use TVWS. The operation and management apparatus is also called an operation and management system, an Operation Administration and Maintenance (OAM) system, or a Central Control Point. The base station information indicates, for example, a geographical location of the base station and height of an antenna utilized by the base station.(2) The GDB determines at least one candidate frequency that can be secondarily used based on the base station information, a frequency band, and a calculation formula of propagation loss, and then informs the operation and management apparatus about the at least one candidate frequency.(3) The operation and management apparatus transfers information of at least one candidate frequency provided by the GDB to the base station.(4) The base station selects a frequency used in its cell (hereinafter referred to as an allocated frequency) from the at least one candidate frequency, based on results of sensing the at least one candidate frequency by a radio terminal (i.e., User Equipment (UE)) belonging to its cell. The base station selects, for example, a candidate frequency on which the smallest interference power measured by the radio terminal as the allocated frequency used in its cell.(5) The base station provides communication services using the allocated frequency.